1. Field of the Invention
Deposits of marine sedimentary fluorapatites are the world's main source of phosphate reserves and resources. As originally laid down on the seabed, this phosphate mineral material is not considered to be simply fluorapatite, Ca.sub.5 (PO.sub.4).sub.3 F, in composition, but rather highly substituted variants of this formula. Thus, it has commonly been found that large amounts of carbonate and lesser amounts of sulfate substitute in the fluorapatite formula for the phosphate and sodium and magnesium ions substitute for the calcium ion. Many substitutions by other elements may also occur, but usually to a lesser extent. Such substituted sedimentary phosphates are known as francolites.
Considerable organic matter was incorporated into these francolite deposits as they were formed on the seabed. Over subsequent geological time, many of these francolite deposits, wholly or at least in part, have become weathered, reworked, or metamorphosed, leading to loss of the organic impurities and other accessory minerals and to changes in the francolite mineral composition towards a less substituted fluorapatite more closely approximating the formula for pure fluorapatite, all such factors leading to a highly desirable enrichment in phosphate content and at least partial removal of undesirable impurities. Commercially exploited phosphate rock deposits representative of such francolite ores are the reworked Bone Valley Formation in Florida, the residual phosphate deposits of Tennessee, and the surficial, more altered and weathered, portions of the Phosphoria Formation in southeast Idaho and surrounding states.
However, the phosphate rock in several deposits which have become commercially viable remains substantially in an unaltered state, with moderate to large amounts of organic matter (organic carbon content of the ore at least 0.5 percent) still present. In such deposits which have not been subject to deep burial during geological time, the francolite mineral retains a high degree of substitution within its crystal lattice by carbonate, sulfate, sodium, etc. Representative of such deposits is the Pungo River Formation of North Carolina.
In contrast to deposits of the North Carolina type, other unweathered deposits have been deeply buried at some stage for long periods of geological time, and the much lower degree of substitution by carbonate found in the structure of such francolites is considered to be caused by burial metamorphism. This process would alter the highly substituted francolite originally deposited to the less substituted, less reactive, and more thermally stable francolite which is present in the deposit today. Representative of this ore type are the unaltered and more deeply buried portions of the Meade Peak member of the Phosphoria Formation, centered in southeast Idaho.
When unweathered rock from both types of deposits discussed above is used in practicing the wet process for effecting the manufacture of phosphoric acid, in which the rock is reacted with a mixture of phosphoric and sulfuric acids followed by filtration of the calcium sulfate formed in the acidulation step to produce a filtrate of phosphoric acid, it has been found that presence of the organic material in the ore causes undesirable foaming and can severely hinder filtration of the byproduct calcium sulfate from the product acid. In order to improve the quality of such rock prior to acidulation, phosphate rocks of both the North Carolina and Idaho types are now generally calcined at a temperature of about 800.degree. C. in a fluidized bed furnace or a rotary kiln to decompose and remove the organic matter.
Incomplete removal of the organic matter during calcination may still cause foaming and filtration problems when the calcine is acidulated in the wet-process phosphoric acid process. In addition, the calcination process unfortunately may increase the sulfide content of the rock. Sulfide in the calcined rock is generally regarded as exacerbating existing filtration problems and of causing marked increases in equipment corrosion when processing rock to wet-process acid.
Particularly objectionable is formation during calcination of what shall, for the sake of convenience, be hereinafter referred to and termed "acid-evolved sulfide," the sulfide which is evolved as a noxious gas such as hydrogen sulfide during acidulation of the rock. In addition to its suspected role in increasing equipment corrosion, acid-evolved sulfide, as a precursor of such toxic fumes, is a potential health and environmental hazard. Acid-evolved sulfide can originate from many sources, usually after reaction of the source material during thermal treatment of the phosphate rock. Sources include sulfur contained in gangue minerals, such as gypsum or pyrite, sulfur exsolved from the francolite crystal lattice during heating of the rock, elemental sulfur impurities, the generally high sulfur content of the organic matter associated with phosphate rock, and sulfur derived from the fuel used in the thermal treatment.
I have found for both the Idaho and North Carolina types of phosphate rock that virtually all the organic impurity and the acid-evolved sulfide content can be eliminated by carefully controlled heating at temperatures of 800.degree. C. or greater for a sufficient period of time in an atmosphere containing excess oxygen. With this procedure, the calcine from Idaho rock still maintains a product of adequate surface area and reactivity for subsequent acidulation processes in manufacturing phosphoric acid or superphosphate. A level of reactivity in the calcine regarded as acceptable to industry corresponds to a surface area in the calcine of at least 2 m.sup.2 /g.
However, when North Carolina rock is calcined under these conditions to eliminate the organic impurity and sulfides, it has been found that the practice, unfortunately, results in a calcined product exhibiting undesirably low surface area (usually about 0.2 m.sup.2 /g), leading to poor reactivity of the rock when acidulated and consequent increased loss of phosphate to the byproduct calcium sulfate filter cake when manufacturing wet-process acid. The differences found in the surface area and reactivity of the North Carolina and Idaho calcines lies in the nature of the francolite mineral in the rocks and their different stabilities towards treatment by heat. Thus, the highly substituted francolite mineral found in North Carolina and similar phosphate rocks is more thermally unstable than the less highly substituted francolite mineral in the Idaho rock, and in contrast to the latter, shows appreciable structural collapse and resultant loss of surface area and reactivity when heated to about 800.degree. C.
It is thus apparent that a need exists for an improved thermal or calcination process for phosphate rocks in which the predominant phosphate mineral is a highly substituted and thus less thermally stable francolite. More specifically, a need exists for a thermal or calcination process for North Carolina phosphate rock where levels of acid-evolved sulfide and residual organic matter in the calcined rock can be minimized but where the surface area of the calcined phosphate can advantageously be maintained at levels greater thant 2 m.sup.2 /g to effect a calcined rock possessing improved and sufficient reactivity to acid attack in the wet process for manufacturing phosphoric acid.
2. Description of the Prior Art
The prior art shows that an attempt has been made to develop such a process for North Carolina phosphate rock. In U.S. Pat. No. 4,348,380, Kenneth L. Parks, a process is disclosed wherein the North Carolina rock is calcined in two stages to retain adequate surface area in the calcine while attempting to minimize residual organic carbon and sulfide levels. Although improvement over a single-stage process was demonstrated, it is apparent from the examples given therein that appreciable amounts of organic carbon and sulfide remained in the calcine under conditions necessary in his two-stage process to yield a calcine of adequate surface area. Thus, in an illustration given of that process wherein a one-hour treatment at 635.degree. C. was followed by a second calcination treatment for 30 minutes at 702.degree. C., the calcined product possessed an adequate surface area of 2.2 m.sup.2 /g, but still contained substantial levels of organic carbon (0.07 percent) and sulfide (0.10 percent). A further drawback of the process is the economic penalty incurred by the requirement that the rock be twice heated to relatively high temperatures.
An approach to phosphate rock beneficiation which has been used for phosphate rocks containing high concentrations of calcite, for example, as described in British Pat. No. 731,999, S. Davidson, June 15, 1955, and No. 1,045,607, E. R. Herman, Oct. 12, 1966, and by A. Talmi et al in Bull. Res. Council of Israel, Vol. 10C, pp. 144-158 (1962), and P. C. Good in U.S. Bureau of Mines Report of Investigation RI 8154 (1976), entails heating the rock at high temperatures wherein the calcite decomposes to lime, i.e., at least 850.degree. C. and usually 900.degree.-1050.degree. C. and subsequently removing the lime by either wet- or dry-slaking processes. Inasmuch as North Carolina feed-grade phosphate rock consists of the aforementioned carbonate-substituted apatite and contains negligible free carbonate accessory minerals, such a process is not of obvious benefit and has not been previously considered for North Carolina phosphate rock. Indeed, at the high temperatures shown by the prior art to be necessary for calcite or calcium carbonate decomposition to CaO, I have found that the surface area, and thus the reactivity, of North Carolina rock calcined at such temperatures is unacceptably low even after treatment with water. None of these previous calcination-slaking processes has addressed the serious problems of residual organic matter, acid-evolved sulfide generation, or loss of rock reactivity which are encountered with calcination of North Carolina rock and similar types of phosphate ores. Therefore, it is evident that the need remains for an improved process which will produce an acceptable calcined product from North Carolina phosphate rock.